Importance of macro-diversity in relay networks has been analyzed and discussed previously in the literature. Macro-diversity may be achieved by broadcasting the data to some relay stations (RSs) (which form a virtual antenna array), one (or some) of which can then forward it to a mobile station (MS). Since the relay stations are distributed spatially, it is assumed that at least one of the relay stations will have sufficiently good channel with both the base station (BS) and the mobile station even when the mobile station is in motion.
Network coding has been employed in a relay network so that the mobile stations can achieve more reliable macro-diversity with the help of a relay-based handover technique. Each mobile station receives the data from the home base station, as well as a network-coded version of the data from the neighboring base station through a relay station. This helps the mobile station while switching from the home base station to the neighbor base station.
A cooperative macro-diversity system for mobile cellular networks has been considered, where the mobile station communicates with the nearest base station directly, and with other base stations through multi-hop relaying (i.e., through another relay station). Compared to a system with single-hop macro-diversity (i.e., the mobile station receives multiple signals directly from all the base stations and there is no relay station assisting in communication), the outage performance and bit error probability can be improved considerably.
The techniques discussed above do not address how multiple transmitters get synchronized before transmitting the packets to the mobile station. Also, they do not consider Multicast and broadcast services (MBS) scenarios.
MBS in today's wireless systems (e.g., IEEE 802.16j wireless standard) typically require synchronization of multiple base stations and/or relay stations in order to achieve macro-diversity. This requires pre-transmission of the packets to the relay stations for synchronization purposes. However, the pre-transmission phase may take longer for some of the relay stations whose number of hops to the base station may be larger than the other relay stations. Therefore, some of the relay stations that are closer to the base station in the tree structure may have to wait in an idle state for the pre-transmissions to propagate to all of the relay stations and until the synchronization is achieved. Then, at a pre-determined target transmission frame, the packets are transmitted synchronously to the mobile station.
Two different techniques are proposed in the IEEE 802.16j baseline document for the synchronization of the relay stations and the base station in MBS. First, the synchronization is achieved by using pre-defined relative transmission times. Each of the relay stations first report its processing delay, DR, to the multi-hop relay (MR)-base station as a capability parameter. The MR-base station then determines the maximum cumulative delay, DM, of all the relay stations, based on their individual delays and their positions in the tree. Then, the waiting times Wi of each of the relay stations (i.e., the waiting time before transmitting synchronously to the mobile station) are calculated and notified to the relay stations.
As an alternative, in the second technique, the MBS data synchronization is achieved using only the target transmission times, which is still determined based on the parameter DM. In other words, rather than the relative waiting time, the relay stations are notified about the absolute transmission time where they should transmit to the mobile station. Note that in both of these synchronization approaches, pre-transmissions may be handled in a unicast or multicast manner.
While synchronization techniques for simultaneous transmission of the packets to the mobile station are presented in these two techniques, none of the two techniques employ ARQ in the relay links to improve the reliability. In other words, when a packet is corrupted, it is simply discarded at the relay station. If this relay station is serving to a large number of mobile stations, the discarded packets will not be received by any of the mobile stations served by this relay station. Also, with these two techniques, some of the relay stations (closer to the base station in the tree) have to wait for the pre-transmissions to propagate to all the relay stations.
A reliable multicast method has been proposed for a relay network, where some critical relay stations are allowed to send back ACK/NACK messages in a multicast connection. The state diagram of a conventional ARQ mechanism is changed so that, the transmitter requires to get ACK/NACK from all the critical relay stations before the state of a packet goes into the “Done” state. While it improves the overall capacity by transmitting to multiple destinations at the same time, it can slow down some relay stations that are successfully receiving the packets, but has to wait for re-transmissions of some other relay stations. Also, some criteria for the selection of critical relay stations are proposed; these criteria are chosen specifically to address multicast connections that use an ARQ mechanism, and not unicast connections.